1. Field of the Invention
The present invention relates mainly to a rolled web of optical film, in which an optical film having an optical orientation axis is rolled up into a roll, and a method of producing the rolled web of optical film, and more specifically relates to a rolled web of optical an, such as polarizing film, optical compensation film, etc., having an optical orientation axis oriented at a predetermined angle, and a method of producing the rolled web of optical film.
2. Description of the Related Art
In recent years, an optical film laminate having various kinds of optical films laminated together is used as a constitutional element of a liquid crystal display device. For example, an optical film laminate, in which a polarizing film that makes only the light vibrating in a specific direction pass therethrough is laminated with various kinds of optical compensation films that improve the drawback of visibility due to the birefringent property of a liquid crystal cell to be used, and the like, are known.
Optical films for use in these optical film laminates each have an optical orientation axis. When in producing an optical film laminate, the respective optical films must be laminated to each other to have an optical orientation axis of one optical film oriented at a predetermined angle relative to the optical orientation axis of the opposite optical film.
A generally employed method of producing an optical film laminate includes cutting sheets of film from an optical film web that is subjected to uniaxial stretching or biaxial stretching to have an optical orientation axis substantially parallel to or substantially orthogonal to the longitudinal direction of the film web, and laminating the sheets of film to each other to have an optical orientation axis of one sheet oriented at a predetermined angle relative to the optical orientation axis of the opposite sheet.
Also, a study is also made on a method of producing an optical film laminate by laminating plural optical films respectively having different angles of optical orientation axis. For example, as described in Patent Document 1, a study is made on a method, which uses a diagonally stretched film obtained by stretching a film web at a predetermined angle relative to the longitudinal direction of the film. The diagonally stretched film can be provided with an optical orientation axis corresponding to a desired angle by changing the stretching angle, and therefore it is known a method in which an optical film laminate is produced by preparing a diagonally stretched film for each of the desired angles of the optical orientation axis and laminating those prepared films together.    Patent Document 1: Japanese Patent Application Laid-open No. Hei-11-231129
However, the former method requires an operation of laminating one by one sheet members cut from an optical film web that has been prepared by uniaxial stretching or biaxial stretching, when in preparing an optical film laminate, which poses a problem of making it difficult to produce optical film laminates in mass quantities, and a problem of deteriorating yield due to the non-laminated areas caused depending on the size of sheet members or angle of laminating the sheet members.
On the other hand, a diagonally stretched film described in the aforesaid Patent Document 1 poses a problem of causing variation in angle of the optical orientation axis. Specifically, a diagonally oriented film, which is stretched by using a tenter-stretching machine to obtain a desired angle of the optical orientation axis, is easy to be influenced by a tension (tensile force) applied to a film before it is stretched, or fluctuation in stretching angle. The diagonally stretched film is curved when it is rolled up into a roll, and hence is partially stretched or shrunken. Because of this, the rolled film web is easy to cause an axial displacement of the optical orientation axis throughout the entire area of the rolled film web, and thus is difficult to produce a rolled web of optical film with a stabilized quality.
Because complicated settings must be made for a diagonally stretched film in terms of stretching angle (direction), stretching force or the like according to the desired angle of the optical orientation axis, preparation of plural optical films respectively having different angles of the optical orientation axis is very troublesome.
Because it is very difficult to form a diagonally stretched film with a desired width in a stretching step, when it is laminated with other film (e.g., a different optical film or adhesive film), it is necessary to prepare a diagonally stretched film having a width greater than the other film and cut the diagonally stretched film to the width of the other film, which invites deterioration in yield.